Wildfires can start when lightning strikes or when someone fails to put out a campfire. New research shows that people start a lot more fires than lightning does — so much so that people are drastically altering wildfire in America.

Fire ecologist Melissa Forder says about 60 percent of fires in national parks are caused by humans: “intentionally set fires, buildings burning and spreading into the forest, smoking, equipment malfunctions and campfires.”

But the average for all forests is even higher. The latest research shows that nationwide, humans cause more than 8 in 10 — 84 percent.

“We are playing a really substantial role in shifting fire around,” says fire ecologist Jennifer Balch at the University of Colorado. Balch looked at the big picture, going through records of 1.5 million wildfires over a 21-year period. She says people are starting fires where and when nature normally doesn’t — at times when forests are often too wet to burn easily or at places and times when lightning isn’t common.

As a result, Balch says, not only are people causing the vast majority of wildfires, they’re also extending the normal fire season around the country by three months. “I think acknowledging that fact is really important,” she says, “particularly right now when we have evidence that climate is changing, and climate is warming, and that fires are increasing in size and the fire season is increasing.”